Differential masculinization of the chelipeds in two simultaneously hermaphroditic Apseudes tanaidaceans (Crustacea: Malacostraca)

Abstract

Simultaneous hermaphrodites are uncommon in Malacostraca, having been reported only in Decapoda, Isopoda, and Tanaidacea. Sexual traits not directly related to reproduction, such as masculinization of chelipeds, have received little attention. This study demonstrated that the tanaidacean Apseudes nipponicus Shiino is simultaneously hermaphroditic; and investigated the allometry of chelipeds in two simultaneously hermaphroditic tanaidaceans, A. nipponicus and Apseudes sp. The two species show differential masculinization of the chelipeds. In A. nipponicus, increase in the widths of the chelipedal carpus and chela relative to body size is diphasic, with a higher growth rate in larger individuals. Large individuals also develop ventral spiniform processes on the carpus and a dorsal triangular process on the fixed finger. In Apseudes sp., increase in the widths of the chelipedal carpus and chela relative to body size is monophasic; large individuals fail to develop ventral processes on the carpus but do produce a dorsal process on the fixed finger. The most masculinized state of cheliped development in Apseudes sp. resembles the state in A. nipponicus individuals at an early stage of chelipedal masculinization. Apseudes sp. is smaller in size than A. nipponicus, suggesting that the former might be a progenetic simultaneously hermaphroditic species, achieving earlier maturation as a female by reducing resource allocation to male secondary traits and growth.